Catalytic hydrodesulfurization of sulfur-containing petroleum hydrocarbons is well known in the art. Generally, for desulfurization, an alumina base catalyst support is used with cobalt, molybdenum and/or nickel added as the catalytically active agents. The hydrodesulfurization reaction involves conversion of the sulfur molecules of the petroleum hydrocarbons to easily removable compounds, such as H.sub.2 S, and this conversion takes place on the active surface of the catalyst. Sulfur-containing petroleum hydrocarbons in many instances also contain heavy metals which during the hydrodesulfurization process, hereinafter referred to as "HDS process", tend to interfere with the efficiency of the catalyst employed. To overcome the effect of the heavy metals, the use of a two-stage system has been recommended, wherein in the first step, the heavy metals are removed by using a catalyst possessing a high macroporosity; and in the second stage, the demetallized hydrocarbons are subjected to hydrodesulfurization using an active desulfurization catalyst.
To overcome the difficulties associated with this dual system, U.S. Pat. No. 3,898,155 recommends the use of a single catalytic system employing a catalyst composition comprising an alumina base support in combination with a Group VI-B metal and at least one Group VIII metal. The catalyst composition is characterized by an average pore diameter greater than 100 angstrom units, a pore volume wherein 10-40% of the total pore volume is in macropores and 60-90% of the total pore volume is in micropores. According to this patent, this catalyst composition is capable of accomplishing simultaneous demetallization and hydrodesulfurization due to the specific pore volume distribution and pore diameter range. In U.S. Pat. No. 3,907,668 an alumina base catalyst composition containing molybdenum, cobalt and/or nickel is described for the HDS process, which is characterized by a pore volume of at least 0.5 cc/g, a surface area of at least 150 m.sup.2 /g and an average pore radius of 45 to 75 angstrom units. At least 65% of the pore volume of the catalyst is in pores having a radius in the range of 50-300 angstrom units.
These prior art patents not only indicate the need of a catalyst composition which can be readily employed in the HDS process, preferably in a one-stage demetallization and hydrodesulfurization process, but also the preferred properties, such as pore diameters in a certain range and pore volumes predominantly in the micro and/or intermediate pore size range.
U.S. Pat. No. 3,907,668 also describes a method of making the alumina base for the HDS catalyst. The method involves the careful calcination of an alumina containing 0-50% by weight bayerite, generally at 427.degree.-871.degree. C (800.degree.-1600.degree. F), followed by impregnation with the catalytic agents. Subsequent to impregnation, the composite is dried, then calcined again at 427.degree.-871.degree. C. This composite is then ready for use for the hydrodesulfurization of petroleum distillates.
Although the results in the above patent indicate a superiority of the described catalyst composition over prior art catalyst compositions, the utilization of bayerite in the precursor of the alumina support imparts certain detrimental effects upon calcination of the precursor. It is known that bayerite, which is a beta alumina trihydrate, converts to the eta alumina form upon calcination at temperatures in excess of about 300.degree. C; and conversion of eta alumina to a more stable theta phase alumina occurs only at temperatures in excess of about 860.degree. C. Eta phase alumina is known to age and exhibits reduced activity, particularly when the catalyst composition is exposed to high temperatures during use or regeneration. Loss of catalytic activity in hydrodesulfurization is indicated by loss of sulfur conversion capacity. In addition, it is also known that the internal porosity of crystalline bayerite is relatively low and when it is used in a catalyst support, its presence can reduce the overall pore volume of the support. These two properties can limit the use of catalyst supports containing eta phase alumina generated by calcination of the bayerite-containing catalyst support precursor.
For catalytic applications, including hydrodesulfurization, alumina supports are preferred which possess stability in combination with high pore volume. This combination insures high catalytic activity, coupled with long service life.
A catalyst support which exhibits thermal stability under extended use at high temperatures is described in U.S. Pat. No. 3,928,236 to Rigge et al. The catalyst support, forming the subject matter of the co-pending application, is prepared by treating alumina shapes having a crystalline phase structure selected from the group consisting essentially of chi-rho-eta, pseudoboehmite and mixtures thereof, with an aqueous ammonium salt solution having a pH within the range of 4-9. Suitable salts disclosed include ammonium acetate, bicarbonate, carbonate, chloride, formate, nitrate and sulfate. The treated shapes are then subjected to a pressure treatment for several hours at 100.degree.-250.degree. C, followed by a water-leach of the pressure treated shapes. Drying of the shapes, followed by calcination in the range of 500.degree.-980.degree. C, produces a thermally stable catalyst substrate. This substrate is suitable for use in many applications; however, when employed as a catalyst support for the HDS process its pore volume distribution does not provide the presently desired, narrow range, particularly in the micro and/or intermediate pore diameter range desired for simultaneous demetallization and hydrodesulfurization.
It has now been unexpectedly discovered that a catalyst support, exhibiting the desired pore volume distribution and preferred pore diameter range for the HDS process can be made by treating an alumina precursor, such as employed in U.S. Pat. No. 3,928,236, with an aqueous solution of ammonium carbonate and/or ammonium bicarbonate, followed by a pressure treatment at a temperature not exceeding about 160.degree. C. The treated alumina is then optionally leached with water, followed by drying and a thermal activation treatment in the range of about 550.degree. and 700.degree. C. The aforedescribed process results in an alumina catalyst support which has a total pore volume in excess of about 0.50 cc/g, wherein at least about 70% of the total pore volume is represented by pores having a pore diameter in the range of about 45-300 angstrom units. This support, when combined with molybdenum, cobalt and/or nickel salts, forms an effective and stable HDS catalyst capable of converting to H.sub.2 S in excess of about 80% by weight of the sulfur content of middle and heavy petroleum fractions for extended periods without losing activity.